Diamines and polyamines of the diphenylmethane series (MDA) are understood to be amines and mixtures of amines of the following formula (I):
wherein n represents a natural number ≥2.
For compounds and mixtures of compounds with n=2, the term “monomeric MDA” (MMDA) is also conventionally used, whilst compounds and mixtures of compounds with n>2 are conventionally referred to as “polymeric MDA” (PMDA). For the sake of simplicity, mixtures containing compounds with n=2 and n>2 side by side are hereinafter referred to as MDA (diamines and polyamines of the diphenylmethane series). Also for the sake of simplicity, the isomers of MMDA are in the following simply referred to as X,Y′-MDA, X, and Y being numerals indicating the substitution pattern of the two phenylene rings.
The most important isomers of MMDA are 4,4′-MDA, 2,4′-MDA and 2,2′-MDA:

4,4′-MDA is sometimes called the para-isomer, whereas both 2,4′-MDA and 2,2′-MDA individually or grouped together are sometimes referred to as ortho-isomers. As a synonym for the PMDA, the terms “higher homologues of MDA” and “oligomers of MDA” can be found in the literature.
MDA is an extremely suitable starting material from which—optionally after further purification—the respective di- and polyisocyanates (hereinafter MDI) that represent an important raw product for polyurethane systems, for example, can be obtained by phosgenation. At the same time, the aliphatic systems that are obtained from MDA by hydrogenation of the aromatic ring also play an important role as paint resins.
Of the many conceivable methods described in the literature for the production of MDA, manufacture from the aniline-formaldehyde condensation product (known as aminal) is the most important because it is the most economically advantageous. This process can be illustrated in idealised form by means of the following diagram:

Depending on the variant, the condensation product (the “aminal”) is produced first and then rearranged in the presence of a catalyst; alternatively, the condensation itself is performed in the presence of a catalyst under rearrangement conditions.
The rearrangement is catalysed by acids. Usually, solutions of strong acids, such as hydrochloric acid, sulphuric acid, and/or phosphoric acid, are employed in either variant, giving rise to the formation of an amine salt, which generally is subsequently neutralized with a base. For this purpose, strong bases such as sodium hydroxide are frequently used. This process suffers from several disadvantages:
Obviously, large quantities of strong acid are required, which is undesirable from an economic as well as an ecological perspective. In addition, use of strong acids may necessitate use of corrosion resistant materials in the equipment. Such construction materials are often expensive. Furthermore, neutralization of the strong acids employed with bases inevitably leads to the formation of large quantities of salts, which must be disposed of safely. These salts may also be contaminated with organic products, which need to be discharged, resulting in increased production costs. Additionally, substantial quantities of waste water are generated by this process, requiring additional processing capacity for the further treatment of the waste water before this can be safely discharged into a sewerage system.
A whole series of suggestions for the industrial implementation of the rearrangement has therefore already been made in order to overcome these disadvantages. Generally, the approach is taken to substitute mineral acids such as hydrochloric acid by solid acids, thereby simplifying the separation of MDA and catalyst as well as, at least in principle, allowing to re-use the catalyst.
However, a feasible method for the production of MDA avoiding mineral acids must meet the following conditions, for example:    a) Quantitative yields: an intermediate-free (aminobenzylaniline-free) product must be obtained in order to ensure that it is capable of being phosgenated (these can be extremely troublesome in the subsequent processing of the MDA to MDI (phosgenation).    b) Isomer distribution: similarly to the mineral acid-catalysed method, the product composition must be able to be controlled to some extent by varying the process parameters.    c) Service life: a catalyst used in industry must achieve an economic service life with high space-time yields before its activity can be restored by means of regeneration.    d) Foreign substances: the catalyst used must release no trace components in the product that have a negative influence on product quality. In addition, the method must cause no foreign matter, e.g. in the form of a solvent that is foreign to the system, to be brought into the reaction mixture.
Clays such as attapulgite and kaolin are stable up to 180° C. and can be regenerated through calcination (cf U.S. Pat. Nos. 4,039,580, 4,092,343 and 4,294,987). However, in the production of MDA they exhibit a low selectivity to the 4,4′-isomer (the ratio of 4,4′-MMDA to 2,4′-MMDA being approximately of from 2 to 4), as a result of their weak acidity. The intolerance to water in the feed (max. 0.15 wt.-%) is a further drawback in industrial application, since a distillation of the intermediate aminal to reduce the water content is cost-prohibitive.
In contrast, amorphous silica-alumina (ASA) materials provide higher activities, an increased water tolerance (up to 3 wt.-%), and stronger acid sites, leading to improved selectivity, the ratio of 4,4′-MMDA to 2,4′-MMDA being approximately 5 (cf U.S. Pat. Nos. 3,362,979, 3,971,829 and BE 1013456A6). However, an even higher ratio of para to ortho isomers would be desirable.
BE 1013456 A6 describes ASA catalysts exhibiting an overall molar silica:alumina ratio of 10 to 500. Catalysts with a lower overall molar silica:alumina ratio are not described as suitable catalysts for MDA synthesis. BE 1013456 A6 pays no attention at all to the significance of the quotient of the molar silica:alumina ratio on the catalyst surface and the overall (bulk) molar silica:alumina ratio.
ASA catalysts are also mentioned in G. Wegener et al., Applied Catalysis A: General (2001) 221, 303-335.
It is known from U.S. Pat. No. 3,860,637 that rearrangement of the aminal using amorphous silicon-aluminium-mixed oxide cracking catalysts results in high yields of 4,4′-isomers when the reaction is performed in the presence of added ortho-isomers. These preferentially react to higher oligomers of MDA. A high proportion of PMDA is therefore conventionally obtained, which has to be separated from the desired 4,4′-isomer. This process requires the additional step of recycling the ortho-isomers initially formed.
DE 2 308 014 A1 describes the synthesis of monomeric MDA over a solid acid catalyst bed in the presence of water. Zeolites, silica-alumina and clays are described as solid catalysts.
U.S. Pat. No. 4,172,847 describes a process for the separation of low functionality substantially pure diaminodiphenylmethanes containing increased 4,4′-isomer contents from methylene-bridged polyphenylpolyamine mixtures prepared by the catalysed condensation reaction of aniline and formaldehyde carried out in the presence of a silica-alumina catalyst. The process is carried out in two steps with the second step below 150° C. Diatomaceous earths, clays and zeolites are described as catalysts.
A general problem of catalysts in MDA synthesis is deactivation due to inefficient removal of reaction products (Alberto de Angelis et al., Ind. Eng. Chem. Res., 2004, 43, 1169-1178). Accordingly, highly mesoporous silica-alumina samples such as MCM-41 have been tested (Carlo Perego et al., Appl. Catal., A, 2006, 307, 128-136). However, selectivity was not sufficiently high, and the synthesis of the catalysts is costly.
M. Salzinger describes in Catalytic methylenedianiline synthesis on porous solid acids (PhD thesis, Technische Universität München, 2010, Sig. 0001/DM 28664) batch and continuous tests on ordered mesoporous aluminosilicates. It is concluded that non-microporous materials deactivate ca. 10 times slower than microporous zeolites. Catalyst deactivation is attributed to polymeric species on the surface.
M. Salzinger et al. describe in Reaction network and mechanism of the synthesis of methylenedianiline over dealuminated Y-type zeolites, Green Chemistry, Vol. 13, No. 1, 2011, 149-155, the results of their investigation on the reaction mechanism of the synthesis of methylenedianiline (MDA) from the condensation product of aniline and formaldehyde (aminal) on microporous acidic materials. The publication does not disclose that the performance of catalysts with a comparatively low molar ratio of silica/alumina on the catalyst surface can be improved if modified such that the quotient of the molar ratio of silica/alumina on the catalyst surface and the overall (bulk) molar ratio of silica/alumina becomes comparatively high.
Jesús Sanz et al. describe in Extraframework Aluminium in Steam- and SiCl4-dealuminated Y Zeolite, J. Chem. Soc., Faraday Trans. I, 1998, 84(9), 3113-3119 the results of a study on the dealumination process of a Y zeolite. The publication does not disclose that the performance of catalysts with a comparatively low molar ratio of silica/alumina on the catalyst surface can be improved if modified such that the quotient of the molar ratio of silica/alumina on the catalyst surface and the overall (bulk) molar ratio of silica/alumina becomes comparatively high.
WO 2010/019844 discloses the application of solid acid silica-metal oxide catalysts in the synthesis of MDA. The conversion is below 100% with ABA concentration of >1%. WO 2010/019844 does not disclose that the performance of catalysts with a comparatively low molar ratio of silica/alumina on the catalyst surface can be improved if modified such that the quotient of the molar ratio of silica/alumina on the catalyst surface and the overall (bulk) molar ratio of silica/alumina becomes comparatively high.
Silanized solid materials having a spaciousness index between 2.5 and 19 are disclosed in EP 1 355 874 B1 as catalysts for MDA synthesis. The ratios of 4,4′- to 2,4′-MDA described therein are, depending on the catalyst, between 1.15 and 3.7. Silanization is rather ecologically unfriendly since the utilized precursor tetraorthosilicate have to be produced from pure silicon over silicon tetrachloride as intermediate (Inorganic Silicon Compounds, W. Simmler in Ullmann's Encyclopedia of Industrial Chemistry, 2000, DOI: 10.1002/14356007.a24_001).
EP-A-0 264 744 describes the condensation of aniline with trioxane or free formaldehyde and the rearrangement to MDA using solid boron, titanium and iron-containing zeolites. Simultaneous condensation and rearrangement as well as isolation of aminobenzylanilines with subsequent rearrangement to MDA are both disclosed. Although high monomer selectivity was obtained by rearrangement of the intermediate aminobenzyl anilines to MDA (approx. 90 mol % MMDA in the product after removal of aniline), complete conversion is not achieved. Furthermore the reaction is preferably performed in an additional solvent which is undesirable from an economic perspective.
WO/0158847 A1 describes a process for the production of MDA containing high amounts of MMDA with low ortho-content via a solid acid-catalysed rearrangement of a condensation product from aniline and formaldehyde or another methylene group-supplying agent like trioxane or para-formaldehyde. Preferred solid catalysts are FAU zeolites. The invention is directed at a process which produces an MDA with as little PMDA as possible. However, whilst very high monomer contents might be desirable for certain special applications, it is not desired as a rule in industry to avoid the formation of PMDA since the latter has proven to be useful in many applications. In addition, the process requires the use of highly pure aniline with a very low content of aliphatic amines.
The solid acid catalysed MDA synthesis is reported in JP 2012 250971 A. Silica-alumina and Y zeolites are described as solid catalysts. However, the conversion of the intermediate aminobenzylanilines to MDA is incomplete.
Through delamination of layered, template-containing zeolite precursors through swelling agents and ultrasound, the acid sites can be made accessible to bulkier molecules, and diffusion limitations avoided (Pablo Botella et al., Appl. Catal., A, 2011, 398, 143-149, WO 03/082803 A1). The catalytic activity of these zeolites with respect to aminal conversion employing a molar ratio of aniline to formaldehyde (hereinafter “A/F”) of 3 is described. The MDA thus obtained contains approximately 25% of PMDA. The exfoliation process results in reduced acid strength compared to zeolitic materials (cf. Avelino Corma et al., Microporous Mesoporous Mater., 2000, 38, 301-309) and thus in a low 4,4′-MDA/2,4′-MDA ratio. Furthermore, this approach is limited to layered zeolites, whose synthesis relies on the application of sacrificial templates and surfactants. The relatively high cost of the zeolites, combined with the excessive consumption of surfactants in the delamination process, render an industrialization of this approach unattractive.
Attempts have also been made to perform the rearrangement of aminal via aminobenzyl aniline to MDA in several steps, for example in two steps, using solid acids in more than one step. U.S. Pat. No. 4,039,581 describes the rearrangement of the aminal using solid acids, whereby the aminal is first dried and then rearranged using zeolites, for example, in several reaction stages with increasing temperature. A temperature of 100° C. is not exceeded. It is assumed that high temperatures in the presence of water would be damaging to selectivity. A full rearrangement of the aminobenzyl aniline intermediates to the MDA cannot be achieved under these conditions. MDA with an MMDA content of approx. 90 mol % in the aniline-free mixture is obtained as product.
WO 2010/072504 A1 describes a continuous process for the synthesis of MDA using solid catalysts. Besides others, different types of silica-alumina as well as clays serve in the first stage to convert aminal towards intermediates like aminobenzylanilines. In the second stage, MMDA and higher homologues are obtained by treating the intermediate mixture with solid catalysts like zeolites, delaminated zeolites or ordered mesoporous materials. The yield of monomeric MDA decreased in general by increasing run-time depending on the choice of catalyst. The necessity of using two different types of catalysts renders the process undesirably complicated.
WO 2016/005269 A1 and T. C. Keller et al., ACS Catalysis (2015) 5, 734-743 describe the use of hierarchical zeolite catalysts for MDA synthesis.
To summarise, there has been considerable progress in the area of solid acid catalysis for MDA synthesis. However, up to now no process running with traditional HCl catalysis on a large industrial scale has been replaced by solid acid catalysis, indicating that further improvements in the field of solid acid catalysis are desirable. It would be particularly desirable to provide a solid catalyst with a flexible compositional window that enables the production of various MDA types. Moreover, reproducibility of catalyst performance features and easy scale-up of the catalyst from the laboratory to industrial scale are desirable.